Generally, metallic materials, for example, stainless steel and titanium metal and titanium alloys, ceramic materials, for example, hydroxyapatite (HAP), bioactive-glass, alumina, and zirconia have been used as biomaterials in practical use. The metallic materials have a good strength and good toughness (tolerance of catastrophic fracture), but have poor corrosion resistance for use in living body. There is a danger of damaging the living tissue because of an elution of metallic materials in living body. The hydroxyapatite and the bioactive-glass have a property of bonding to a living bone (that is to say bioactivity), so that the hydroxyapatite and the bioactive-glass are turned to practical use as bone compensatory materials, a periodontal filling materials, an artificial vertebra body, and any spacer. These materials have a strength and a toughness that is much less than that of a living bone, so that these materials have not been used as portions that is under high load-bearing conditions, e.g. a femoral bone and tibial bone, and an artificial root of a tooth. As compared with a hydroxyapatite and bioactive-glass, alumina has a high strength and a high toughness. However, application of the alumina and the zirconia as a bone repairing material is limited because of non-bioactivity. Alumina had been turned to a practical use as an artificial root of a tooth, but now the alumina was replaced by titanium metal and titanium alloys because of fragile property peculiar to the ceramics.
As described above, the metallic materials, e.g. titanium metal and titanium alloys, and the ceramics materials, e.g. alumina and zirconia are non-bioactive materials that cannot bond to the living bone. Preferably, the bioactive layer that has a good adhesive property to bone are formed, and the bioactive function are given on the surface of the non-bioactive materials, in order that the materials are used as a bone repairing material and an artificial root of a tooth. Several prior art methods of forming the bioactive layer on the materials have been disclosed. For example, a method of forming a layer on base materials by a sputtering process or evaporation process is disclosed. The Japanese Laid-open Patent Publication No. 4-242659 discloses a method of forming a layer on base materials by plasma spraying. The Japanese Laid-open Patent Publication No. 1-203285 discloses a method of forming the mixture of zirconia and apatite on the surface of a zirconia cast by coating and sintering, with the base material limited to zirconia in view of the high strength and high toughness.
By the way, several prior art methods of giving the bioactive function at the surface of a base materials are disclosed. The Japanese Laid-open Patent Publication No. 6-23030 discloses a method of forming a coating layer of silica gel or titania gel on the surface of a base material. The Japanese Laid-open Patent Publication No. 10-179718 discloses a method of improving the surface of a base material of titanium metal and titanium alloys to bioactive by soaking in an alkaline fluid.
To form the bioactive layer on the surface of the base materials, the layer by sputtering, evaporation, and plasma spraying cannot have good contact strength to the base materials. Regarding with the layer formed by the Japanese Laid-open Patent Publication No. 1-203285, the bioactivity is declined because of increasing the ratio of zirconia in the mixture of the zirconia and the apatite, while the adhesive strength is declined because of increasing the ratio of apatite in the mixture. The layer having the hydroxyl group formed by the method of the Japanese Laid-open Patent Publication No. 6-23030 are the silica gel layer or the titania gel layer on the surface of the base materials. Similarly, the bioactive layer formed by the method of the Japanese Laid-open Patent Publication No. 10-179718 are titania phase, titania gel phase, alkaline-titanate phase, and alkaline-titanate gel phase.